Question

Based on the color of the solutions, determine the direction of the reaction equilibrium shift upon addition of each reagent investigated and write the chemical reaction involving the reagent that is responsible for the shift. Each of the following reagents was added to FeSCN2+ (aq) and the color change is indicated. I'm not sure of the product side of each reaction, please help.

a) Fe(NO3) + FeSCN2+ turned from red to dark red (This one might not have an equation)

b) NH4SCN + FeSCN2+ red turned black

c) SnCl2 + FeSCN2+ red turned cloudy orange

d) AgNO3 + FeSCN2+ red turned cloudy white

e) NaCl + FeSCN2+ red turned orange

f) NaOH + FeSCN2+ red turned dark yellow

g) Na2HPO4 + FeSCN2 red turned clear

Answer 1

Generally we can say that;

An increase in the concentration of the reactants shifts the equilibrium in the forward direction and a decrease in the concentration of the reactants shifts the equilibrium in the backward direction.

Similarly, an increase in concentration of the products shifts the equilibrium in the backward direction and a decrease in the concentration of the products shifts the equilibrium in the forward direction.

a)     Fe(NO₃)₃ + Fe[SCN]²⁺ turned from red to dark red:

If we add ferric nitrate to the solution of ferric thiocyanate complex, the concentration of ferric ions increases. Increasing the concentration of reactant ions the equilibrium shifts toward the product side. Therefore the complex gets dark red color.

b)    NH₄SCN + Fe[SCN]²⁺ red turned black:

Here SCN^- ion concentration increases in the reactant side therefore the equilibrium shifts towards product side.

c)     SnCl₂ + Fe[SCN]²⁺ red turned cloudy orange:

Here Fe³⁺ ion concentration decreases, as it reacts with chloride ion to form FeCl₃. Therefore equilibrium shifts towards reactant side.

d)    AgNO₃ + Fe[SCN]²⁺ red turned cloudy white:

Here Fe³⁺ ion concentration decreases, as it reacts with nitrate ion to form Fe(NO₃)₃. Therefore equilibrium shifts towards reactant side.

e)     NaCl + Fe[SCN]²⁺ red turned orange:

Here Fe³⁺ ion concentration decreases, as it reacts with chloride ion to form FeCl₃. Therefore equilibrium shifts towards reactant side.

f)      NaOH + Fe[SCN]²⁺ red turned dark yellow:

Here Fe³⁺ ion concentration decreases, as it reacts with hydroxide ion to form Fe(OH)₃. Therefore equilibrium shifts towards reactant side.

g)     Na₂HPO₄ + Fe[SCN]²⁺ red turned clear:

Here Fe³⁺ ion concentration decreases, as it reacts with hydrogen phoshate ion to form Fe₂(HPO₄)₃. Therefore equilibrium shifts towards reactant side.